Optical glass fibers for use in transmitting light (hereinafter referred to as "light-transmitting optical glass fibers" or simply as "optical glass fibers") are brittle, easily flawed or cracked, and have poor flexibility. The formation of such flaws or cracks leads to easy breakdown of the optical glass fibers even when slight external force is applied thereon. Certain resin coatings, therefore, have heretofore been applied to the surface of the optical glass fibers immediately after the production thereof.
Resinous materials which have heretofore been used in the resin coating include epoxy resin and urethane resin. These resinous materials, however, have disadvantages in that productivity when using them is low, since a long period of time is required for curing, the adhesion to glass fibers is insufficient, and the long term reliability is not satisfactory. Furthermore, since the fiber coatings prepared using such resinous materials have poor flexibility, light-transmitting characteristics are deteriorated by microbending.